This proposal concentrates on the control of seated posture and balance for individuals with low tetraplegia or paraplegia through the application of electrical stimulation. It represents an innovative approach to an unaddressed need of persons with spinal cord injuries and an advance in motor system neural prostheses, which have traditionally focused on upper and lower limb function. Lack of control of the spinal muscles restricts an individual's ability to vary seated position or maintain balance while reaching or manipulating objects in the environment. Moreover, long-term paralysis results in unhealthy postures that lead to skeletal deformities, increases risk for decubitus ulcers, and imposes pressures that compromise internal organ function. This project will determine the feasibility of a closed-loop control system for trunk posture. Through a coordinated series of computer simulations with a musculoskeletal model of the human torso, experimental verification with able bodied volunteers, and testing with subjects with low cervical or high thoracic spinal injuries, an automatic control system based on sensors for trunk orientation and acceleration will be developed to allow paralyzed users to assume and maintain trunk positions away from the backrest. This involves: a. Extending and improving an anatomically-based model of the biomechanics of the trunk and performing meaningful simulations. b. Verifying the model using EMG recordings and kinematic/kinetic data from able-bodied subjects in different trunk postures. c. Using the computer model to design controllers to allow a patient to set trunk position with a feed-forward input device like a joystick and maintain balance using feedback from accelerometers and other body-mounted sensors. d. Testing the controller on current users of neuroprostheses employing electrical stimulation.